The well known mobile radio system GSM is a TDMA-system (Time Division Multiple Access) designed for frequency hopping.
TDMA involves the time-wise division of a carrier wave frequency into a number of time slots, eight time slots/carrier wave in respect of the GSM-system. The eight time slots recur at given intervals and each recurring time slot is corresponded by a radio channel.
By a frequency hopping system is meant a system in which a radio channel changes its carrier wave frequency during an ongoing connection. In the case of the GSM-system, the carrier wave frequency is switched between each channel time slot. Normally, switching of a carrier wave frequency normally takes place in accordance with a predetermined scheme, a so-called hop sequence. In the case of a frequency hopping system, a channel is defined by its allocated hop sequence and time slot, whereas a channel is defined by its carrier wave frequency and time slot in the case of a non frequency-hopping system.
Fading is a problem with respect to radio communication and occurs as a result of multipath propagation of the radio wave between a transmitter and a receiver. This means that in certain positions in space, the various wave propagation paths co-act to obtain a high signal strength, whereas immediately adjacent signals having different wave propagation attenuate each other so as to result in a low signal strength. Positions with low signal strengths are called fading dips. Data that is sent to a mobile station, which is located in a fading dip, will often be lost. In the case of a mobile radio system that has a relatively narrow bandwidth, such as the GSM-system, fading dips will occur at different positions for different carrier wave frequencies.
The problem associated with fading dips is reduced by frequency hopping a channel. The risk of the mobile station being located within a fading dip during the course of several time slots then no longer exists, whilst the likelihood of individual time slots being knocked out by a fading dip increases at the same time. By coding user data and interleaving the coded data, i.e. cut up sequences of the data and mixing the same with other sequences over a longer period, the user data can be recovered in a receiver even though individual time slots have been lost.
Frequency hopping, however, requires the use of advanced equipment. Those mobile stations that were produced when the GSM-system was new were unable to make frequency hops. Consequently, several years passed before operators introduced frequency hopping in their respective networks.
Fixed frequency planning is applied in existing GSM-systems, i.e. each cell included is allocated a number of carrier wave frequencies with corresponding radio channels in order to be able to serve mobile stations within the cell with communication. The GSM-system was originally adapted to be a telephony system that transmitted speech between users. In the case of speech connections, it is important that the information between the terminal users is not delayed, since it would then not matter so much if the information is somewhat distorted. User data, i.e. speech that is transmitted over the radio connection, is always sent channel coded in accordance with a predetermined coding scheme. Quality is measured in order to maintain satisfactory quality on the radio connection. On downlink, i.e. on the link from a base transceiver station BTS to a mobile station MS, quality is measured in the mobile station. The results of the measurements are then sent in reports to the base transceiver station BTS. On the uplink, i.e. the link from the mobile station to the base station, quality measurements are made in the base station. The base station is connected to a base station controller. The results of the measurements are evaluated in the base station controller and controlled so as to maintain correct quality on the radio connection. The base station controller includes two devices for effecting this control, these devices being one for varying the transmitted radio power and another for switching radio channels. Radio channel switching can be effected within the own cell and is designated intracell handover. Intracell handover is normally implemented when the received signal strength is high but the channel is disturbed by another transmitter, so-called interference. Radio channels can also be switched between cells, designated intercell handover. This procedure is used when the mobile station is mobile and enters a new cell. The signal strength on the old channel/cell is thus impaired, and, at the same time, improves on a channel in the new cell.
In addition to the aforesaid telephony service, a packet data service designated GPRS (General Packet Radio Service) is now being standardised for the GSM-system and TDMA/136. As the name implies, GPRS is designed to send packet data between two users. As distinct from speech in respect of telephony, user data in respect of packet data transmission is not particularly sensitive to delays, but, on the other hand, is highly prone to lose information. Data sent from a transmitter to a receiver is divided into blocks. Each block is provided with check bits, which enable it to be ascertained whether the block detected is correct, or not. When the block detected is found to be erroneous, the receiving equipment requests the transmitting equipment to re-transmit the block. A mobile station may be a transmitter, a receiver or both and at the same time, i.e. a transceiver.
A logic function designated Link Adaptation checks and controls the transmission over the radio connection; i.e. adapts the radio link in accordance with the prevailing radio environment. There is a significant difference between link adaptation for GPRS and the control of a radio connection in telephony. In the case of telephony, it is necessary for transmission quality to exceed a lowest threshold in order to be acceptable. If this cannot be achieved on the channel used at that time, the channel is changed. The purpose of GPRS Link Adaptation is to maintain the highest possible data rate for user data on an allocated radio channel. Data rate is influenced by the extent to which the ambient radio environment interferes with the radio channel in question. The data rate is decreased when it is necessary to send data blocks as a result of the blocks not being received correctly. The number of symbols that are changed on a radio channel that is subjected to a difficult radio environment will be more than the number of symbols changed on a channel that is subjected to a simpler radio environment.
There are four alternative channel coding schemes for coding user data that is transmitted over the radio link. The first coding scheme CS1 applies to each bit of user data a single bit coding, i.e. the user data in the transmitted coded data has a proportionality of 1:2. CS1 is intended for the most difficult radio environment. The second and third coding schemes CS2 and CS3 have a respective proportionality of ⅔ and about ¾ on the user data in the coded data, and are adapted for correspondingly less harsh interference on the radio channel. According to the fourth coding scheme CS4, the user data is not coded at all, i.e. the user data proportionality is 1/1. CS4 is preferably only used when the radio environment is very good.
The purpose of link adaptation is to evaluate the quality of a radio connection to which a given radio channel has been allocated and to select a coding scheme with which the highest possible data rate is obtained. If data is transmitted with insufficient coding, the proportion of wrongly detected frames will increase, meaning that these frames must be re-transmitted and therewith result in a lower data rate. If an excessively strong coding is used, the data rate will be reduced by virtue of the fact that the capacity available is not used for user data. Switching between coding schemes takes place dynamically during an ongoing connection, in response to changes in the radio environment. The criteria which determine when a coding scheme change shall take place have been pre-selected.
When a new connection is established, transmission is effected initially with either CS1 or CS2. Estimation of the radio quality is then also commenced and a switch to a coding scheme, which involves a lower coding proportionality, i.e. to CS3 or CS4, is made when the result of the measurements indicates that this is necessary.
When the radio environment worsens, a coding scheme change is effected according to GPRS, such as to increase coding of the user data. The telephony service of the GSM-system on the other hand would have carried out an intracell handover if the quality of the radio connection were to fall beneath a given value. Consequently, intracell handover is not a term in GPRS. It is, however, technically possible to effect a change of radio channel within the cell. A channel change is called a Packet Timeslot Reconfiguration TS GSM 04.60.
EDGE is another new standard within the GSM-system and TDMA/136, which was still not fully specified at the time of filing of the instant patent application. EDGE is based on GPRS but makes a still higher data rate possible through the medium of a further modulation method. The introduction of EDGE provides eight different alternative radio channel modes, designated MCS1–MCS8. The first four modes MCS1–MCS4 of these eight modes conforms with the GPRS modulation method and, in principle, also to the coding schemes CS1–CS4. The four new modes MCS5–MCS8 have, in principle, the same type of coding as CS1–CS4, but use 8PSK instead of GSMK as a modulation method and therefore give twice the data rate.
In a scientific document entitled “Capacity Evaluation of the EDGE Concept for Enhanced Data Rates in GSM and TDMA/136, A. Furuskär, M. Höök, S. Jäverbring, H. Olofsson, J. Sköld, published at the conference “IEEE Vehicular Technology Conference 99”, a comparison is made of data rates on block levels for the different modes of EGPRS/?/with and without frequency hops. EGPRS is not defined, but is thought to correspond to EDGE. It is said that the link quality control scheme used lowers the performance of the higher rates with frequency hops. According to the document, however, this difference is expected to decrease with current link quality control schemes that reduce the depth of interleaving.
In another document “Comparison of Link Quality Control Strategies for Packet Data Services in EDGE” 0-7803-5565-2/99, published at the same time as and by the same authors as the above-mentioned document, (radio) link quality control is considered and suitable block lengths for achieving high data rates are discussed. It is proposed that the blocks are divided into sub-blocks in respect of the modes MCS-8 that enable the highest possible data rates to be achieved. One of several reasons for dividing a block into sub-blocks is that a fading dip in a frequency hop need only knock out a sub-block instead of an entire block.
U.S. Pat. No. 5,095,500 describes how the geographical position of a mobile station can be determined with the aid of signals transmitted over a radio channel. This technique, however, has hardly any relevance to the present invention.